The present invention relates to a UV lamp for operating radiation with wavelengths in the ultraviolet spectral range and with an optical path in which a filter material of doped quartz glass is provided.
Moreover, the present invention relates to a method for irradiating a surface, a liquid or a gas with UV radiation.
UV lamps in the form of discharge lamps, such as deuterium, halogen, excimer or mercury-vapor lamps, are for instance used for medical and therapeutic applications, in material processing, for sterilization and in spectroscopic devices. These UV lamps have a UV radiation source for emitting UV radiation. UV radiation encompasses the wavelength range of 100 nm to 380 nm according to DIN 5031, Part 7. Before the radiation emitted by the UV radiation source impinges on the item to be irradiated, it passes through a beam exit window. This is a lamp bulb which surrounds the UV radiation source (or a part thereof), or for instance, a cladding tube which surrounds the lamp bulb (or a part of a cladding tube of this nature).
Besides the desired UV operating radiation, the emission spectrum of UV lamps often includes a portion of short-wave ultraviolet radiation having wavelengths below 190 nm that lead to the formation of ozone and can be detrimental to health or induce material to age. Moreover, ozone itself has an absorption band with a maximum wavelength of around 255 nm, which can reduce the intensity of the UV operating radiation.
Quartz glass is transparent to UV radiation over a wide wavelength range and is therefore, in principle, suited as a material for the beam exit window. The transmission of quartz glass depends on the temperature. In the shortwave wavelength range between 140 nm and 200 nm, it is substantially defined by an absorption edge, the so-called Urbach edge (I. T. Godmanis, A. N. Trukhin, K. Hübner “Exciton-Phonon Interaction in Crystalline and Vitreous SiO2”, Phys. Stat. Sol. (b), 116 (1983), 279-287). The Urbach edge shifts with a rising temperature toward longer wavelengths. However, at the typical lamp operating temperatures (above 100° C.), it is still within much shorter wavelengths than 190 nm. It is thus not suited for preventing the formation of ozone.
That is why deuterium lamps often use a lamp bulb consisting of a special borosilicate glass, a so-called “UV glass”, which absorbs radiation with a wavelength below 190 nm. Borosilicate glass, however, does not show a particularly steep absorption profile, but rather the profile flattens towards the longer-wave range, which reduces transmission in the region of the operating wavelength such that a spectral transmission of less than 80% mm−1 is normally obtained at a wavelength around 210 nm.
Deuterium lamps emit UV radiation in the wavelength range of about 180 nm to 380 nm in the form of a continuous, substantially line-free spectrum, and are therefore preferred radiation sources for UV spectroscopy. A high optical stability in the sense of a time-constant emission is demanded from high-quality spectral analyzers. Here, lamp bulbs of borosilicate glass show another weakness. That is, their optical transmission for the UV operating radiation is considerably decreasing in the course of time.
Quartz glass is less prone in this respect. Moreover, quartz glass can be processed more easily than borosilicate glass and exhibits an improved temperature resistance. To reduce the formation of ozone, it is common in deuterium lamps with a lamp bulb of quartz glass to cover the exit window with a multilayered coating which acts as an interference filter for wavelengths below 190 nm. With the help of such interference layers, it is possible to produce a steep absorption edge between a stopband and a passband at a defined edge wavelength. Such a UV lamp is known, for instance, from DE 39 02 144 A1, where the beam exit window is configured over a part of the lamp bulb circumference.
Interference filters of this nature consist of a multitude of thin material layers. The application of the material layers onto the lamp bulb is time-consuming and material-intensive.
It is also known that undesired portions of the UV radiation in the emission spectrum of a lamp are filtered out by doping the glass of the beam exit window with substances that absorb in the wavelength range in question. Titanium dioxide, for instance, is used as a dopant in quartz glass and produces an absorption band with an absorption maximum at about 200 nm. Instead of or in addition to this, International Application Publication No. WO 2010/112311 A1 suggests a filter material consisting of a quartz glass doped with gallium oxide.
In quartz glasses doped with titanium oxide or gallium oxide, the edge wavelength λc is in the wavelength range of 230 to 250 nm. They are not suited as a filter material for an operating radiation in the wavelength range around 210 to 230 nm.
DE 10 2013 204 815 A1 describes rare earth-doped fiber lasers. The increase in refractive index caused by rare-earth doping is compensated by co-doping with fluorine. Doping with rare-earth metal oxide and Al2O3 is carried out at different mole fractions. Rare earth metal-doped glasses of this nature show strong absorptions in the UV and VUV range and are suited as filter glasses for filtering out the UV wavelength range.
EP 0 822 167 A2 is concerned with a preform for optical quartz-glass fibers with an Al2O3-doped fiber core. The homogeneously doped fiber core may contain up to 1.3 wt. % Al2O3. Higher amounts will lead to crystallization during fiber drawing. Fiber cores with a central dip in the Al2O3 concentration can also have higher Al2O3 contents of up to 2.35 wt. % outside their “centerline”.
DE 647 537 C describes transition glasses for producing a melting of tungsten or molybdenum current feeds in lamp bulbs. The glasses contain 65-96% SiO2 and between 4-20% Al2O3 and optionally further oxides. Indicated is a set often transition glasses, whose coefficient of expansion will increase step by step.
U.S. Patent Application Publication No. 2012/0148770 A1 describes packaging glass for pharmaceutical products. The glass contains between 82 and 99.9999 wt. % SiO2, and the residue is formed by dopants, such as Al2O3. The Al2O3-doped quartz glass is distinguished by high chemical resistance and moderate viscosity.
In view of the above-explained weaknesses of former filter materials for beam exit windows of UV lamps, the use of doped quartz glass would per se be an appropriate measure for achieving a filter action in an inexpensive and reproducible manner.
However, suitable dopants are not readily available for producing the desired absorption without having a simultaneous impact on the transparency for the operating radiation. Moreover, doping may cause unwanted changes in the quartz glass properties. Specifically, doping can change the viscosity and thermal stability of the quartz glass, increase its tendency to crystallization and reduce the radiation resistance to UV radiation. The latter has a particularly disadvantageous effect especially in quartz glass for UV lamps, because UV radiation damage causes a gradual decrease in UV transmission (aging) and thus a decreasing UV emission.
It is therefore an objective of the present invention to provide a UV lamp having a beam exit window containing filter material of doped quartz glass, wherein doping, on the one hand, does not significantly impair the resistance of the quartz glass to crystallization and temperature, but, on the other hand, effects maximum transparency for operating radiation in the ultraviolet spectral range above 210 nm together with little transparency in the wavelength range below about 190 nm.
Moreover, it is an objective of the present invention to provide a method for effective irradiation of a surface, a liquid or a gas by means of UV radiation together with minimal ozone formation.